Active termination network

ABSTRACT

An active termination for a transmission line comprising a reference impedance, a terminating impedance and a control circuit. The reference and terminating impedances are identical circuits made on the same integrated circuit in close proximity to one another. Both impedances are made of an active and a passive resistor in series. The active resistor is a CMOS transistor operated as a voltage controlled resistor. A control circuit senses the impedance of the reference impedance and generates a control signal to change the impedance of the reference and terminating impedances such that they are made equal to the impedance of the transmission line. An alternate embodiment of the invention comprises an active resistor and a passive resistor in series to form a terminating impedance network. A control circuit senses the voltage on the transmission line and adjusts the active resistor to terminate the transmission line with the correct value of resistance.

TECHNICAL FIELD

This invention relates generally to an active circuit for terminating atransmission line.

BACKGROUND OF THE INVENTION

Computer and communications systems frequently use transmission lines totransmit data within and to other systems and subsystems. Transmissionlines must be properly terminated to minimize distortion of thetransmitted signals. This is particularly important as the operatingfrequencies of transmission lines are increased to match the higheroperating frequencies of the latest integrated circuits. Many systemsand integrated circuits currently have operating frequencies in the GHzrange.

Terminating networks are used to provide the correct terminatingimpedance. An off-chip precision resistor can provide transmission linetermination. Off-chip resistors require significant circuit board spaceand increase board complexity and cost. The distance between theoff-chip resistors and the transmission line to be terminated can belarge enough to cause signal reflection problems. The use of on-chiptermination resistors is preferred, but high precision resistors aredifficult to form using standard CMOS processes due to manufacturingprocess and thermal variations. A resistor made by current CMOSprocesses could have a tolerance range of as much as 30%, which istotally unacceptable for use as a transmission line terminator. Lasertrimming of on-chip CMOS resistors is expensive and to be avoided, if atall possible.

The use of active circuits, such as FETs (Field Effect Transistors), toterminate a transmission line is known in the art and there have beenmany approaches to this problem. It is also known in the art that a FET(a field effect transistor) behaves like a resistor for low currents andvoltages, but that the channel resistance of a FET is non-linear. Forinstance, U.S. Pat. No. 5,422,608 describes “Adaptive Transmission LineTermination” in which a FET terminates a transmission line and anamplifier is used to control the voltage applied to the gate of the FET,thus allowing its resistance to be matched to the impedance of thetransmission line.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an active circuitfor terminating transmission lines and that may be composed ofimprecise, high tolerance components configured to operate as aprecision termination whose impedance varies only marginally from thecharacteristic impedance of the transmission line terminated.

The present invention improves upon previous attempts to provide anactive termination circuit for terminating transmission lines with ahigh degree of accuracy, especially when made on an integrated circuit.This invention uses a reference impedance, which is a series combinationof an active resistor and a passive resistor. The active resistor is aCMOS transistor operated as a voltage controlled resistor. A variablevoltage is applied to the gate to control the drain to sourceresistance. The passive resistor is set to a fixed resistance.

The current through the reference impedance is set to a fixed value. Acontrol circuit with a feedback loop senses the voltage across thereference impedance to generate a control voltage to control theresistance of the active resistor and thus control the resistance of thereference impedance.

The terminating impedance and the reference impedance are made asidentical circuits formed at the same time on the same integratedcircuit and as a result have identical properties. The terminating andreference impedances are designed to have the same impedance and arecontrolled by the same control voltage generated by a feedback loop.

An alternate embodiment of the invention does not use a referenceimpedance. An active resistor and a passive resistor in series form theterminating impedance circuit. A control circuit senses the voltage onthe transmission line and adjusts the gate voltage to control the activeresistor to terminate the transmission line with the correct value ofresistance.

The present invention has many advantages, which will become clear fromthe detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of a transmission line terminationcircuit in accordance with the invention.

FIG. 2 shows an alternate embodiment in accordance with the invention.

FIG. 3 shows a simulation of comparison of passive, active, andpassive/active combination resistances in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a transmission line 1 terminated at the receiving end byterminating impedance network 8, which is made of an active resistor 9in series with a passive resistor 10. Active resistor 4 and passiveresistor 5 form a reference impedance 3, whose resistance is controlledby a control signal coming from op amp 2. As part of a negative feedbackloop, op amp 2 compares the voltage drop across resistors 4 and 5against a reference voltage Vref to generate a control voltage whichcontrols the drain to source voltages of active resistors 4 and 9, thuscontrolling the channel resistances of 4 and 9. Current source 7 limitsthe current flow through the reference resistance 3.

The present invention relies on the well-known fact that a FET behaveslike a resistor for low currents and voltages, but the channelresistance of a FET is non-linear.

The channel resistance of a FET is non-linear and varies as the drain tosource voltage of the device is changed. Since a FET is used toterminate a transmission line, it is subject to this non-linearitycaused by the voltage swing on the line. In order to reduce this voltagesensitivity, this circuit incorporates a resistor 10 in series with theFET 9. (A POLY resistor is used as resistor 10 in this embodiment, butother resistors can be used.) The nominal resistance of the POLYresistor is chosen to be approximately equal to the nominal FET channelresistance, hence the maximum voltage variation seen across the FET iscut by half and its overall non-linearity contribution is reduced by afactor of 2.

The ratio between the active and passive resistors need not be 1 to 1,but can be adjusted during the design of the active terminator to suit aparticular application.

In a present embodiment, current source 7 limits the current flowthrough the reference resistance 3 to 10 mA. Op amp 2 generates acontrol signal to keep the voltage drop at node 6 to 0.5 volts below thesupply voltage Vdd. As a result, the resistance of 3 is equal to 0.5v/10 ma, which is 50 ohms, which is a common characteristic impedancefor transmission lines. In order for the feedback loop to hold theresistance 3 at 50 ohms, the reference voltage Vref should be 0.5 voltsbelow the supply voltage Vdd. Selection of the specific circuit valueswill depend on the operating conditions of a particular transmissionline. As an example of how this embodiment would function, if the signalon the transmission line is driven by a current source with a currentlimit of 10 mA, then the current source 7 should have a value of 10 mA.The current source should have this value because the more similar theoperating conditions (such as maximum current load) of the terminatingimpedance 8 and reference impedance 3, the more equivalent theirimpedances will be, the importance of which is described below.

Reference resistance 3 and terminating resistance 8 are designed asidentical circuits on the same integrated circuit. Because of theirphysical proximity and creation through identical manufacturingprocesses, they will have virtually identical properties. The closerthat resistances 3 and 8 are to each other on the same integratedcircuit, the more equivalent their thermal environments and performanceswill be. As the control voltage is automatically varied by the feedbackloop to keep the reference impedance 3 at a value of 50 ohms, the samevoltage is used to keep terminating impedance 8 at a 50 ohm value. Thus,the transmission line is terminated with an impedance of the correctresistance.

Parameters can be changed so that the resistance 3 matches whatevercharacteristic impedance the transmission line in use has, making themonitoring of the transmission line voltage in this embodimentunnecessary. This and other circuit design adjustments, such asaccommodating transmission lines driven by a voltage source or havingdifferent termination impedances can be made by those skilled in theart.

This invention is able to provide a termination impedance which iswithin 5% of the characteristic impedance of the transmission line beingterminated. This is a considerably better impedance match (orequivalence) between the terminating impedance and characteristicimpedance of the transmission line than can be achieved with the use ofpassive on-chip resistors alone as terminating impedances, as wasdiscussed in the background section of this application.

This invention can also easily accommodate ratios between theterminating impedance and the reference impedance other than one to one.For example, if it were important to limit the power consumption of anintegrated circuit as much as possible, and the terminating impedancewas 50 ohms, then the reference impedance could be designed to be 500ohms. This would reduce the overall power consumption of the referenceimpedance by 90 percent. Such a modification of the invention can bemade by those skilled in the art.

It is also possible for those skilled in the art, to use this inventionfor terminating balanced transmission lines.

Adapting this invention such that the terminating impedance is in serieswith the transmitting end of the transmission line is well known tothose skilled in the art.

FIG. 2 is an alternate embodiment of the invention, in which a controlcircuit directly controls the terminating impedance.

FIG. 2 shows a transmission line 21 terminated at the receiving end by aterminating impedance 24, which is made of an active resistor 25 inseries with a passive resistor 26. As part of a negative feedback loop,peak detector 22 senses the voltage on the transmission line. The peakdetector 22 then generates a voltage which op amp 23 compares against areference voltage Vref to generate a control voltage. This controlvoltage at the output op amp 23 determines the drain to source voltageof active resistor 25, thus controlling the channel resistance of activeresistor 25. The reference voltage Vref applied to op amp 23 is chosenso that the output of op amp 23 controls the resistance of FET 25 sothat the resistance of the terminating impedance 24 matches theimpedance of transmission line 21.

Voltage level detection circuits such as peak detectors are well knownto those skilled in the art and their design and use need no furtherelaboration.

FIG. 3 shows simulation results comparing resistances of passive,active, and passive plus active resistors over a voltage range. (FIG. 3is simply an example for a specific application to illustrate and verifythe overall usefulness of the invention.) The vertical axis 32 showsresistance in ohms. The horizontal axis 31 shows the voltage variationgenerated by a variable voltage source applied to one end of the devicebeing tested (passive resistor, active PFET or resistor and PFET inseries connection). A 3.3 Volt voltage supply is connected to the otherend of the device being tested in the simulation. The variable voltagesource produces voltages between 2.7 and 3.3 volts, Consequently, thevoltage across the device tested varies between 0.0 V and 0.6 V in thissimulation.

Graph 33 shows the voltage across the 50 ohm passive resistor only asthe voltage across the resistor varies. As expected, the resistanceremains 50 ohms across the voltage range. Graph 34 shows the non-linearresistance of the active PFET 9 only as the voltage across the PFETvaries. Graph 34 in FIG. 3 shows that the resistance of the PFET variesbetween approximately 42 to 62 ohms over a change in the voltage of 0.6volts. The resistance of the PFET exhibits a tolerance range of −16percent to +24 percent as compared to the desired resistance of 50 ohms.Graph 35 in FIG. 3 shows that the series resistance of the PFET and a 25ohm resistor varies between approximately 48 and 53 ohms over a voltagechange of 0.6 volts. This means that the resistance of the PFET inseries with the resistor exhibits a tolerance range of −4 percent to +6percent as compared to the desired resistance of 50 ohms. Thesesimulation results show the superior performance of the seriescombination of the PFET and the resistor as an active impedance.

In actual operation, voltages at the end of the transmission line 1 varywith the bit transmitted on the line. This results in a changing voltageacross termination network 8 and attendant fluctuations in the channelresistance of the PFET 9 depending on the bit being transmitted. As thesimulation shows, use of a passive series resistor with the PFET 9 inthe termination network reduces impedance fluctuations across thetermination network 8 dramatically for a given voltage fluctuation,allowing the impedance of the of the termination network to vary only afew ohms around the characteristic impedance of the transmission line(50 ohms), as is shown by comparing graphs 33 and 35 in FIG. 3. Thus, asgraph 35 shows, the PFET plus resistor network is able to match thecharacteristic impedance of the transmission line 8 (50 ohms) much moreclosely than a termination network consisting of a PFET only,represented by 34 in the graph. The reduced resistance fluctuationreduces signal reflections along the transmission line and allows ahigher range of operating frequencies.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade without departing from the spirit and scope of the invention asdefined solely by the appended claims.

What is claimed is:
 1. A transmission line termination circuitcomprising: a connection to a transmission line; a terminating impedanceconnected to said transmission line through said connection, includingan active impedance and a passive impedance connected in series; areference impedance, including an active impedance and a passiveimpedance connected in series, wherein said reference impedance ismanufactured with properties proportional to the properties of theterminating impedance; a control circuit for generating a control signalfor controlling the impedance of the active impedance of the referenceimpedance, in order for the reference impedance to match thecharacteristic impedance of the transmission line; and said controlsignal controls the impedance of the active impedance of the terminatingimpedance in order for the terminating impedance to substantially equalthe characteristic impedance of the transmission line.
 2. Thetransmission line terminating circuit of claim 1, wherein theterminating impedance is connected in parallel with the transmissionline.
 3. The transmission line terminating circuit of claim 1, whereinthe terminating impedance is connected in series with the transmissionline.
 4. The transmission line terminating circuit of claim 1, whereinthe active impedance of the reference impedance comprises a transistor.5. The transmission line terminating circuit of claim 4, wherein saidtransistor is a field effect transistor.
 6. The transmission lineterminating circuit of claim 1, wherein the active impedance of theterminating impedance comprises a transistor.
 7. The transmission lineterminating circuit of claim 6, wherein said transistor is a fieldeffect transistor.
 8. The transmission line terminating circuit of claim1, wherein the reference impedance and the terminating impedance aredisposed on the same integrated circuit.
 9. The transmission lineterminating circuit of claim 1, wherein the transmission line carries abalanced signal and is terminated by a balanced termination.
 10. Thetransmission line terminating circuit of claim 1, wherein the referenceimpedance is manufactured to have properties substantially identical tothe properties of the terminating impedance.
 11. The transmission lineterminating circuit of claim 1, wherein the ratio of voltage across thereference impedance to current through reference impedance issubstantially constant and substantially equal to the characteristicimpedance of said transmission line.
 12. The transmission lineterminating circuit of claim 1 wherein the ratio of voltage across theterminating impedance to current through the terminating
 13. A method ofcontrolling the active impedance of a terminating impedance forterminating a transmission line, comprising the steps of: sensing thevoltage across a series-connected active resistor and passive resistorof a reference impedance; comparing the sensed voltage to a referencevoltage; generating a control voltage in response to variations in thesensed voltage; varying the resistance of the active resistor of saidreference impedance in response to said control signal, and; varying theresistance of an active resistor of a terminating impedance in responseto said control signal such that the terminating impedance substantiallyequals the characteristic impedance of the transmission line.
 14. Themethod of claim 13 further comprising applying said control voltage tothe gate of a field effect transistor to vary the resistance of thetransistor.
 15. The method of claim 13 further comprising connecting theterminating impedance in parallel with the transmission line.
 16. Themethod of claim 13 further comprising connecting the terminatingimpedance in series with the transmission line.
 17. The method of claim13 further comprising: maintaining the ratio of voltage across thereference impedance to current through reference impedance at asubstantially constant level and substantially equal to thecharacteristic impedance of said transmission line.
 18. The method ofclaim 13 further comprising maintaining the ratio of voltage across theterminating impedance to current through the terminating impedance at asubstantially constant level and substantially equal to the referenceimpedance.
 19. The transmission line terminating circuit of claim 13,wherein the reference impedance is manufactured to have propertiessubstantially identical to the